The invention relates to a method for improving the Bayer process for the production of alumina from bauxite ore. The invention concerns the use of cross-linked polysaccharides, specifically cross-linked dextran or cross-linked dihydroxypropyl cellulose to improve the performance of unit operations within the Bayer process, in particular to enhance the settling of fine alumina trihydrate crystals.
In the typical Bayer process for the production of alumina trihydrate, bauxite ore is pulverized, slurried with caustic solution, and then digested at elevated temperatures and pressures. The caustic solution dissolves oxides of aluminum, forming an aqueous sodium aluminate solution. The caustic-insoluble constituents of bauxite ore are then separated from the aqueous phase containing the dissolved sodium aluminate. Solid alumina trihydrate product is precipitated out of the solution and collected as product.
As described at least in part, among other places, in U.S. Pat. No. 6,814,873, the Bayer process is constantly evolving and the specific techniques employed in industry for the various steps of the process not only vary from plant to plant, but also are often held as trade secrets. As a more detailed, but not comprehensive, example of a Bayer process, the pulverized bauxite ore may be fed to a slurry mixer where an aqueous slurry is prepared. The slurry makeup solution is typically spent liquor (described below) and added caustic solution. This bauxite ore slurry is then passed through a digester or a series of digesters where the available alumina is released from the ore as caustic-soluble sodium aluminate. The digested slurry is then cooled, for instance to about 220° F., employing a series of flash tanks wherein heat and condensate are recovered. The aluminate liquor leaving the flashing operation contains insoluble solids, which solids consist of the insoluble residue that remains after, or are precipitated during, digestion. The coarser solid particles may be removed from the aluminate liquor with a “sand trap”, cyclone or other means. The finer solid particles may be separated from the liquor first by settling and then by filtration, if necessary.
The clarified sodium aluminate liquor is then further cooled and seeded with alumina trihydrate crystals to induce precipitation of alumina in the form of alumina trihydrate, Al(OH)3. The alumina trihydrate particles or crystals are then classified into various size fractions and separated from the caustic liquor. The remaining liquid phase, the spent liquor, is returned to the initial digestion step and employed as a digestant after reconstitution with caustic.
Within the overall process one of the key steps is that of precipitation of the alumina trihydrate from the clarified sodium aluminate liquor. After the insoluble solids are removed to give the clarified sodium aluminate liquor, also referred to as “green liquor”, it is generally charged to a suitable precipitation tank, or series of precipitation tanks, and seeded with recirculated fine alumina trihydrate crystals. In the precipitation tank(s) it is cooled under agitation to induce the precipitation of alumina from solution as alumina trihydrate. The fine particle alumina trihydrate acts as seed crystals which provide nucleation sites and agglomerate together and grow as part of this precipitation process.
Alumina trihydrate crystal formation (the nucleation, agglomeration and growth of alumina trihydrate crystals), and the precipitation and collection thereof, are critical steps in the economic recovery of aluminum values by the Bayer process. Bayer process operators strive to optimize their crystal formation and precipitation methods so as to produce the greatest possible product yield from the Bayer process while producing crystals of a given particle size distribution. A relatively large particle size is beneficial to subsequent processing steps required to recover aluminum metal. Undersized alumina trihydrate crystals, or fines, generally are not used in the production of aluminum metal, but instead are recycled for use as fine particle alumina trihydrate crystal seed. As a consequence, the particle size of the precipitated trihydrate crystals determines whether the material is to be ultimately utilized as product (larger crystals) of as seed (smaller crystals). The classification and capture of the different sized trihydrate particles is therefore an important step in the Bayer process.
This separation or recovery of alumina trihydrate crystals as product in the Bayer process, or for use as precipitation seed, is generally achieved by settling, cyclones, filtration and/or a combination of these techniques. Coarse particles settle easily, but fine particles settle slowly. Typically, plants will use two or three steps of settling in order to classify the trihydrate particles into different size distributions corresponding to product and seed. In particular, in the final step of classification a settling vessel is often used to capture and settle the fine seed particles. Within the settling steps of the classification system, flocculants can be used to enhance particle capture and settling rate.
The overflow of the last classification stage is returned to the process as spent liquor. This spent liquor will go through heat exchangers and evaporation and eventually be used back in digestion. As a result, any trihydrate particles reporting to the overflow in this final settling stage will not be utilized within the process for either seed or product. Effectively such material is recirculated within the process, creating inefficiencies. Therefore, it is important to achieve the lowest possible concentration of solids in the overflow of the last stage of classification to maximize the efficiency of the process.
As described for example in U.S. Pat. No. 5,041,269, conventional technology employs the addition of synthetic water soluble polyacrylate flocculants and/or dextran flocculants to improve the settling characteristics of the alumina trihydrate particles in the classification process and reduce the amount of solids in the spent liquor. While various flocculants are often used in the trihydrate classification systems of Bayer plants, it is highly desirable to reduce as far as possible, the loss of solids with the spent liquor.
Thus there is clear need and utility for a method of improving the classification and flocculation of precipitated alumina trihydrate in the Bayer process. Such improvements would enhance the efficiency of the production of alumina from bauxite ore.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.